Air conditioning is a highly desirable and, in some instances, essential feature in modern automobiles and other vehicles. In a typical installation, a compressor is used to pressurize and propel a volatile refrigerant such as Freon through a closed-loop system. A cooling coil is provided in the closed loop within the passenger compartment of the vehicle. The compressed, liquid phase, refrigerant expands into a gaseous phase as it passes through the cooling coil causing ambient heat to be withdrawn from the surrounding air. The withdrawal of heat cools the vehicle. The heated, expanded, refrigerant is subsequently repressurized by the compressor, causing loss of heat to the outside air. So long as the compressor continues to operate, the refrigerant continues to circulate through the system exchanging heat between the inside and the outside of the vehicle by an associated phase change in the refrigerant.
The air conditioning compressor in most vehicles is powered by direct interconnection with the engine which, in most cases, is an internal combustion engine powered by gasoline, diesel fuel or a similar combustible compound. The compressor is typically interconnected with the engine's crankcase by a power transmission belt provided between a pulley on the compressor and an associated driving pulley on the engine. The driving pulley is, similarly, connected with a variety of other engine components including a water circulation pump, a cooling fan, and an electrical generator or alternator. The engine's driving pulley is usually driven directly from the crankcase and, hence, rotates at a speed that is directly proportional with the speed of engine operation. Thus, when the engine operates at an idling speed of, for example, 1,000 RPM, the air conditioning compressor pulley is driven at a proportionate idle speed. Conversely, when the engine is driven at a cruising speed at, for example, 4,000 RPM, the compressor is driven at approximately 4 times the idle speed.
A typical air conditioning compressor is operated by simply engaging an air conditioning-on switch which causes an electrical clutch in the compressor's pulley to lock the pulley relative to a compressor drive shaft. When the switch is "off", the drive shaft and pulley are disengaged and the pulley simply free wheels relative to the drive shaft. The associated disadvantage to this arrangement is described further below. Nevertheless, the great variation in driving speeds of the air conditioning compressor as engine speed changes, causes a significant variation in refrigerant pressurization. The variation in refrigerant pressurization causes a substantial variation in air conditioning cooling output. For example, at idle, the compressor's cooling output can be approximately 6,000 BTU. When the car is traveling at approximately 3,500 RPM, this output can be approximately 36,000 BTU. Hence, the air conditioner may not provide sufficient cooling at a low RPM, while the cooling is more efficient at cruising speed. At higher speeds, in which engine speed exceeds 5000 RPM, the compressor cannot operate for long periods without becoming damaged. Hence most compressors include a shutoff control that signals the clutch to disengage from the engine. Hence at high speeds the compressor continually cycles on and off. This can lead to discomfort on hot days in which continual cooling is required.
Similarly, it has been shown that the compressor suffers accelerated damage when it is operated at high RPMs. Thus, it is often desirable to deactivate the air conditioning when cruising. This can lead to discomfort on very-hot days.
As discussed above, the operation of the air conditioner compressor's clutch can lead to further damage since, at high RPMs, the sudden activation of the clutch sends a shock through the compressor as it is "slammed on". There is also increased risk of fan belt breakage when the clutch suddenly engages the compressor at higher speed. It would be preferable to power the compressor via a gradual acceleration to a fully-powered state.
It is, therefore, an object of this invention to provide an air conditioning system for vehicles that operates at a substantially constant speed and cooling output regardless of engine speed. It is a further object of this invention to provide an air conditioning system that operates continuously at a speed that minimizes air conditioning compressor damage. It should be possible to activate and deactivate the compressor at will, while the engine is operating at any speed without risk of damage to the compressor or other engine components. Cooling output by the system should not be excessive at high engine speed and should be sufficient at idle and low engine speed.